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Topic: Thermochemistry-Enthalpy Problem  (Read 4185 times)

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Nano

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Thermochemistry-Enthalpy Problem
« on: November 06, 2004, 02:14:04 AM »
Hi,

we are learing thermochemistry right now, and I don't get what enthalpy is, in our book, it says enthalpy is H. When we are calculating Energy, we use Delta E=q+w. I get that part, it means Energy equals heat plus work. But I don't get what enthalpy means, how it works and how to calculate it, i'll apreciate your help

James

Demotivator

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Re:Thermochemistry-Enthalpy Problem
« Reply #1 on: November 08, 2004, 09:39:45 AM »
Delta E=q+w (where E is internal energy).
If a process takes place at constant pressure, then pressure-volume work results if the volume of the system changes. Rewriting the above:
deltaE = qp - P(deltaV)
or,
  qp = deltaE + P(deltaV)

By definition, H=E+PV. Thus, for a process which takes place at constant pressure,
qp = deltaH
This says that the heat exchanged between system and surroundings at constant pressure is the enthalpy. Since most chemical processes occur at constant pressure, enthalpy is a useful function because we only need to measure the heat absorbed or evolved without worring about the pressure-volume work.
« Last Edit: November 08, 2004, 02:12:55 PM by Demotivator »

Nano

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Re:Thermochemistry-Enthalpy Problem
« Reply #2 on: November 09, 2004, 06:24:10 PM »
thanks a lot  :D

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Re:Thermochemistry-Enthalpy Problem
« Reply #3 on: November 09, 2004, 06:54:56 PM »
Remember Thermodynamics is about what can and can not happen. Kinetics is about how fast it happens.
Most Common Suggestions I Make on the Forums.
1. Start by writing a balanced chemical equation.
2. Don't confuse thermodynamic stability with chemical reactivity.
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Nano

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Re:Thermochemistry-Enthalpy Problem
« Reply #4 on: November 14, 2004, 10:20:03 PM »
roger that

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